1. Field of the Invention
This invention relates to solid state relay circuits and, more particularly, to solid state relay circuits with overload and short circuit protection circuitry and additional circuitry to indicate the presence of an overload or a short circuit.
2. History of the Prior Art
A great variety of solid state relay circuits have been developed which use a power semiconductor as the output circuit switching device. A major disadvantage of such circuits has been their sensitivity to current overload or short circuit which may destroy the switching device. For this reason, various circuitry has been devised to turn off the switching devices when overload currents or short circuit conditions occur. Examples of such circuits are disclosed in U.S. patent Ser. No. 4,581,540, entitled Circuitry Overload Protected Solid State Relay, Ciro Guajardo, issued Apr. 8, 1986.
Although such circuits provide appropriate protection against current overloads and short circuits in the load circuitry, the fact that such overloads have occurred is not apparent. A circuit for indicating that an overload or short circuit has occurred is very useful because it allows corrections to be made to the circuitry or its environment to eliminate the condition.
It is, therefore, an object of this invention to provide improved solid state relay circuits.
It is another object of this invention to provide circuitry indicating that solid state relay circuits incorporating circuitry for protecting against overload currents and short circuits in the load circuit have, in fact, responded to overloads or short circuits.